Radio frequency power amplifiers are devices typically comprising a large RF power amplifier transistor that amplifies a RF signal and transfers the RF signal to a load termination through an impedance matching circuit. RF power amplifier transistors are typically integrated in an active die which is typically arranged in a package.
FIG. 1 schematically shows a cross-section of a typical package 1 used in prior art solutions for RF power amplifier of base stations of a suitable communications network. FIG. 2 shows a top view of the same package 1. The cross-section shown in FIG. 1 is taken along the line I-I schematically drawn in FIG. 2. The package 1 comprises a metal flange 2 with on a top side thereof an electrically isolating window frame 3 attached thereto. The flange 2 and the window frame 3 form a cavity. In the cavity, an active semiconductor die 4 is situated on the top side of the flange 2. The package 1 comprises an input lead L1 and an output lead L2 fixated on the isolating frame 3 at respective opposite lateral sides L1,L2, such that the leads extend outwards in a lateral direction and overlap with a top surface of an external printed circuit board PCB, when the flange 2 is positioned in a cavity or window of the PCB. The leads L1, L2 are connected with bondwires G1 and D1 to a corresponding input terminal and to a corresponding output terminal of an RF power amplifier transistor integrated in the active die 4. The active die 4 may comprise, additionally to the RF power amplifier transistor, active or passive components such as for example integrated inductors or capacitors for impedance matching. Although not shown in FIGS. 1 and 2, the open top side of the package is typically closed off, e.g. by a mold compound.
Typically, the metal flange 2 of the package 1 is bolted down to a heat sink 5 (or base plate 5) while the input lead L1 and the output lead L2 may be soldered to the external printed circuit board PCB. The heat sink 5 (or base plate 5) may also be part of the printed circuit board PCB on which the package 1 is mounted. Typically, the heat sink 5 may be electrically in contact with a conductive bottom ground layer 6 of the printed circuit board PCB. A bottom side of the active die 4 is typically attached to and physically in contact with the top side of the metal flange 2. A reference terminal of the RF power transistor is connected to the bottom side of the active die 4, which in turn may be connected to the ground potential via the metal flange 2 and the heat sink 5. The metal flange 2, the heat sink 5 and the bottom ground layer 6 of the printed circuit board PCB thus provide a path to ground for the RF power transistor.
However, this typical package suffers from instability, as well as poor predictability of input and output matching impedances.